Many classes of electromagnetic device require initialization. An example is the Frequency Selective Limiter (FSL), a magnetostrictive device which is used as an amplitude compressor in communication systems. Examples of FSL's appear in U.S. Pat. Nos. 4,595,889 and 4,845,439 to Stitzer et al. The FSL's dipole domains tend to spin synchronously with the frequency of signals input to the device, and when driven into a non-linear regime, the FSL dissipates energy into its substrate and thus reduces the amplitude of signal at that frequency. This causes the FSL to act as a narrow band notch filter about a specific resonant frequency, and some FSL's have quite narrow linewidths. This in turn makes them valuable as narrowband filters such as are needed for channelized communication systems, e.g. receivers for radio, television, cable television, electronic warfare command and control systems, etc. Further, the dipole domains saturate at a specific input power. This causes the FSL to attenuate a larger signal more strongly than a smaller one. Together, these two attributes have resulted in FSL's being used as amplitude compressors, i.e. devices that narrow the amplitude gap between larger undesired signals and smaller desired ones, permitting easier detection of the desired ones. (Or, more to the point, permitting detection of the smaller desired signal possible with less complicated and expensive hardware.)
FSL's have a significant drawback. In order to be effective, an FSL's dipole domains must "spin up," i.e. become energized from their relaxation state. Before spin up, the FSL is largely ineffective, nominally for a time of 100-300 ns. For simplicity, systems with FSL's typically use the leading edge of the input signal to spin up the FSL's dipole domains. This means that the energy in the leading edge of the input signal is transmitted unattenuated, and thus no compression in the output signal results. This is of little consequence for continuous wave communication systems. It is, however, of considerable consequence for digital systems, particularly those whose inter-pulse period is of the same order as the spin-up time, about 10.sup.-7 sec and above, or about 0.01 GHz and below. In the spin-up period, FSL spin-up can cause a significant loss of data. The preferred operating range of the FSL comprehends a large portion of the RF spectrum, generally 2-6 GHz.